Method and apparatus for optical signal analysis using a gated modulation source and an optical delay circuit to achieve a self-homodyne receiver

ABSTRACT

An actual optical field spectrum is mixed down to baseband and can be observed directly to measure the optical field spectrum of an optical signal based on a local oscillator approach. This local oscillator approach is achieved without adding an additional local oscillator. A single optical source, such as a laser, is controlled by a modulation source for selectively modulating the optical signal produced by the optical source. Accordingly, the optical signal produced by the optical source alternates between two states, namely, an unmodulated state and a modulated state which carries the optical field spectrum of interest. Preferably, the optical signal produced by the optical source is routed to an optical conduit in parallel with an optical delay line. Although the two states of the optical signal produced by the optical source occur sequentially in time, they are mixed together concurrently after being channeled through the parallel optical circuit comprising the optical conduit and the optical delay line. The unmodulated state of the optical signal produced by the optical source serves as a local oscillator signal. The parallel combination of the optical conduit and the optical delay line together with a photodetector functions as a self-homodyne receiver when fed the unmodulated and modulated states of the optical signal produced by the optical source. The self-homodyned mixing of the unmodulated and modulated states of the optical signal produced by the optical source serves to frequency-translate the optical power spectrum to within the bandwidth of an analyzer, such as a microwave spectrum analyzer.

BACKGROUND OF THE INVENTION

This invention relates to optical spectrum analysis and, moreparticularly, to measurement of the spectrum of a modulated opticalsignal. Specifically, the invention is directed to a method andapparatus for measurement of the spectrum of a modulated optical signalusing a gated modulation source and a self-homodyne detection methodbased on the use of an optical delay circuit for recovering the opticalfield spectrum on a modulated optical signal.

The power spectrum of an optical source determines the performance ofany optical device, such as a fiber optic system or associatedcomponent, that operates on that source. For example, if a lasertransmitter feeds into an optical fiber cable for a link to an opticalreceiver elsewhere in a fiber optic system, the power spectrum of thelaser determines the amount of pulse distortion due to dispersion in theoptical fiber and thus the effectiveness of the communication link.

Various techniques for measuring this power spectrum are known.Unfortunately, they all have limitations and/or disadvantages inperforming power spectrum measurements.

One known technique involves the use of a grating spectrometer. However,in practice, resolution requirements often exceed those possible with agrating spectrometer.

Other known techniques employ Fabry Perot, Mach Zehnder, or Michelsondiscriminators. However, the presence of AM confuses measurementsperformed with these discriminators.

Another known technique utilizes a scanning Fabry Perot spectrometer.However, this spectrometer has limited dynamic frequency range ifoperated over a wide spectral band.

Finally, a technique for synthetic heterodyne interferometry forsemiconductor laser spectral analysis is disclosed in Abitbol, C.,Gallion, P., Nakajima, H., and Chabran, C.: "Analyse la LargeurSpectrale d'un Laser Semiconducteur par Interferometrie HeterodyneSynthetique," J. Optics (Paris), 1984, Vol. 15, No. 6, pp. 411-418. Thelaser is frequency shift keyed by superimposing a small amplitudesquare-wave signal on a bias injection current. The optical field isanalyzed by an unbalanced Mach Zehnder single-mode fiber interferometerwhich includes an optical delay circuit. A detector at the output of theinterferometer acts as an optical product detector. Unfortunately, themodulation is constrained to be a square wave, and the modulation rateis tied to the delay in the optical circuit, so that the square wave hasa period of twice the delay.

SUMMARY OF THE INVENTION

In accordance with a preferred embodiment of the invention, a method andapparatus are provided based on a local oscillator approach so that theactual optical field spectrum is mixed down to baseband and can beobserved directly. Preferably, the method and apparatus of the inventionachieve this local oscillator approach without adding an additionallocal oscillator.

One embodiment in accordance with the invention provides directmeasurement of the optical field spectrum of an optical signal. Themethod and apparatus of the invention use only a single optical source,such as a laser, controlled by a modulation source for selectivelymodulating the optical signal produced by the optical source.Accordingly, the optical signal produced by the optical sourcealternates between two states, namely, an unmodulated state and amodulated state which carries the optical field spectrum of interest.

Preferably, the optical signal produced by the optical source is routedto an optical conduit in parallel with an optical delay line. The timeduration of each state equals the delay of the optical delay line or aninteger fraction thereof. Although the two states of the optical signalproduced by the optical source occur sequentially in time, they aremixed together concurrently after being channeled through the paralleloptical circuit comprising the optical conduit and the optical delayline. The unmodulated state of the optical signal produced by theoptical source serves as a local oscillator signal. The parallelcombination of the optical conduit and the optical delay line togetherwith a photodetector functions as a self-homodyne receiver when fed theunmodulated and modulated states of the optical signal produced by theoptical source. The self-homodyned mixing of the unmodulated andmodulated states of the optical signal produced by the optical sourceserves to frequency-translate the optical power spectrum to within thebandwidth of an analyzer, such as a microwave spectrum analyzer.

The method and apparatus of the invention are essentially wavelengthindependent (limited only by detectors and fiber components >300 nm).Additionally, no tracking of an additional local oscillator is required.Furthermore, unlike known optical spectrum analysis systems, includingthe one disclosed in the aforementioned Abitbol, et al., article, highfrequency modulation is applied when the modulation source is gated onby a gate function to provide gated modulation of the optical signalproduced by the optical source, i.e., there is modulation under the gatefunction. This modulation can be at any frequency greater than the gatefrequency, rather than the modulation rate being tied to the delay ofthe optical delay line.

BRIEF DESCRIPTION OF THE DRAWINGS

The above and other features of the invention and the concomitantadvantages will be better understood and appreciated by persons skilledin the field to which the invention pertains in view of the followingdescription given in conjunction with the accompanying drawings. In thedrawings:

FIG. 1 is a schematic diagram of an optical spectrum analysis system inaccordance with one embodiment of the invention;

FIG. 2 illustrates a signal produced by a gated modulation sourceincluded in the optical spectrum analysis system shown in FIG. 1;

FIG. 3, comprising FIGS. 3A and 3B, shows frequency spectra for anunmodulated optical signal (FIG. 3A) and a corresponding modulatedoptical signal (FIG. 3B);

FIGS. 4A and 4B illustrate an optical signal produced by an opticalsource included in the optical spectrum analysis system shown in FIG. 1as alternating between two states, namely, an unmodulated state (FIG.4A), which serves as a local oscillator signal, and a modulated state(FIG. 4B), which carries the optical field spectrum of interest;

FIGS. 5A and 5B show a microwave spectrum analyzer output for sinusoidalmodulation, f_(m) =100 MHz, m=83% (FIG. 5A), and NRZ pseudorandom bitsequence (PRBS), f_(c) =350 MHz, m=20%, length=2⁷ -1 (FIG. 5B), in testson a DFB laser; and

FIG. 6 shows the relationship between maximum frequency chirp (at 1.32um) and modulation depth for sinusoidal (f_(m) =300 MHz) and PRBS (f_(c)=365 MHz) modulation in tests on a DFB laser.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

The invention allows direct measurement of the optical field spectrum onan optical signal. A single optical source, such as a laser, is used tofrequency-translate the optical power spectrum to within the bandwidthof an analyzer, such as a microwave spectrum analyzer. This is achievedby alternating, or switching, the optical signal produced by the opticalsource between two states, namely, an unmodulated state, which is usedin lieu of a local oscillator signal, and a modulated state, whichcarries the optical field spectrum of interest, and by mixing thesestates in a self-homodyne receiver which includes an optical delaycircuit.

Considered in more detail, FIG. 1 is a schematic diagram of an opticalspectrum analysis system in accordance with one embodiment of theinvention, generally indicated by the numeral 10. The optical spectrumanalysis system 10 comprises an optical source 12 for producing anoptical signal. The optical source 12 can be, for example, a laser, suchas a DFB laser.

In accordance with the invention, the optical spectrum analysis system10 further comprises a modulation source 14 connected to the opticalsource 12. The modulation source 14 is preferably a gated modulatorwhich produces bursts of modulation. Accordingly, as shown in FIG. 2,the modulation source 14 generates a gate function 16 which is employedas a trigger signal to gate a modulation signal 18, thereby producing agated modulation signal 20. The modulation signal 18 on which the gatefunction 16 is superimposed can be continuous wave, pulse, pseudorandombit sequence (PRBS), or other type of modulation. The gated modulationsignal 20 preferably has a 50% duty cycle and a period 2T, as shown inFIG. 2. The relationship between the frequency of modulation and gatefrequency is:

    f.sub.m >f.sub.GATE

The gated modulation signal 20 produced by the modulation source 14modulates the optical signal produced by the optical source 12. As shownin FIG. 3, the optical signal produced by the optical source 12alternates between two states, namely, an unmodulated state, as shown inFIG. 3A, and a modulated state which carries the optical field spectrumof interest, as shown in FIG. 3B.

Referring again to FIG. 1, the optical spectrum analysis system 10further comprises a first optical power splitter 22 connected to theoptical source 12. The optical spectrum analysis system 10 alsocomprises an optical conduit 24, whose input is connected to the firstoptical power splitter 22, and an optical delay line 26, whose input isalso connected to the first optical power splitter. The optical conduit24 can be optical fiber cable or, alternatively, the atmosphere, i.e.,free space. Preferably, the optical delay line 26 comprises apredetermined length of optical fiber cable. The relationship betweengate frequency and the predetermined time delay of the optical delayline 26 is:

    f.sub.GATE ·T=n+1/2, where n=0, 1, 2 . . .

The output of the optical conduit 24 and the output of the optical delayline 26 are connected to a second optical power splitter 28 alsoincluded in the optical spectrum analysis system 10. Therefore, theoptical conduit 24 and the optical delay line 26 are connected in aparallel optical circuit.

The optical spectrum analysis system 10 further comprises aphotodetector 30. For example, the photodetector 30 can be a photodiode.Preferably, the detection bandwidth of the photodetector 30 is widerthan the AM and FM bandwidths. Finally, the optical spectrum analysissystem 10 preferably comprises an analyzer 32, such as a microwavespectrum analyzer or RF spectrum analyzer.

The combination of the first optical power splitter 22, optical conduit24, optical delay line 26, second optical power splitter 28, andphotodetector 30 forms a self-homodyne receiver in response to thealternately unmodulated and modulated optical signal produced by theoptical source 12 shown in FIG. 3, as will now be described. Theunmodulated optical signal produced by the optical source 12 is used inlieu of a local oscillator signal.

Let E_(A) (t) be the optical signal which passes through the opticalconduit 24 and E_(B) (t) be the optical signal which passes through theoptical delay line 26. An illustration of E_(A) (t) appears in FIG. 4Aand of E_(B) (t) appears in FIG. 4B. E_(A) (t) and E_(B) (t) alternatelyserve as the local oscillator signal and the modulated optical signal.

As a result of the splitting and recombining of the alternatelyunmodulated and modulated optical signal produced by the optical source12 in the first and second optical power splitters 22 and 28, the fieldE_(O) is composed of two parts, one due to the modulated optical signalproduced by the optical source and another due to the unmodulatedoptical signal. Therefore, the signal current ID in the photodetector 30is proportional to:

    E.sub.A (t).sup.2 +E.sub.B (t).sup.2 +2[E.sub.A (t)·E.sub.B (t)]

The first two terms represent optical intensity modulation. The lastterm gives the spectral information of interest.

As an example, if for a time T, as shown in FIG. 4, E_(A) (t) is theunmodulated optical signal produced by the optical source 12 currentlyserving as the local oscillator signal, and E_(B) (t) is the modulatedoptical signal with both AM and FM imparted on it, then the third term(which is proportional to E_(A) (t)·E_(B) (t)) in the above expressioneffectively represents the mixing between the equivalent of a localoscillator signal and the unknown signal. This results in a measurementwhose resolution is a function of the unmodulated linewidth of theoptical source 12, such as a laser, and the interferometer transferfunction of the optical conduit 24 and the optical delay line 26.

The optical spectrum analysis system 10 can directly measure the chirp alaser undergoes during IM modulation. The laser modulation is preferablygated by a square wave of period twice the differential delay of theoptical circuit. When the gate function 16 is on, modulated signal isallowed to pass to the laser. Thus, the result is that two opticalbeams, one unmodulated and the other modulated, are mixed, therebyyielding a term associated with laser chirp directly.

For example, as DFB semiconductor lasers become more commonplace inoptical communication systems, there exists a growing need for theiraccurate characterization. Measurements of linewidth and small frequencydeviations have been demonstrated by taking advantage of the long delaysachievable using fiber optic circuits. See Okoshi, T., Kikuchi, K., andNakayama, A.: "Novel Method for High Resolution Measurement of LaserOutput Spectrum," Electron. Lett., 1980, Vol. 16, pp. 630-631, and Ryu,S., and Yamamoto, S.: "Measurement of Direct Frequency ModulationCharacteristics of DFB-LD by Delayed Self-Homodyne Technique," Electron.Lett., 1986, Vol. 22, pp. 1052-1054.

Another important measurement, useful in determining the dispersion overa fiber link, is that of the spectrum of the optical field of a currentmodulated DFB laser. The method and apparatus in accordance with theinvention use the optical delay line 26 combined with gated modulationto measure the homodyne power spectrums of both the optical field andintensity of a DFB laser under normal operating conditions.

In accordance with the method and apparatus of the invention, the mixingof a modulated DFB laser with a CW local oscillator is achieved with asingle laser by using gated modulation combined with an appropriateoptical delay to perform frequency chirp measurements up to +/-22 GHz.Detecting the mixed optical signal with a wide bandwidth analyzer 32(i.e., optical detector, preamplifier, and microwave spectrum analyzer)allows direct observation of frequency chirps.

Considered in more detail, a DFB laser provides the optical source 12and is switched between two states of operation, each state lasting atime T. In one state, the laser operates as a CW local oscillator. Inthe other state, any desired AC-coupled modulation can be applied to thelaser. The time T is assumed to be much longer than the period of themodulation.

The laser signal is then fed to the first optical power splitter 22 and,hence, to the parallel optical circuit comprising the optical conduit 24and the optical delay line 26, whose differential time delay T resultsin the continuous addition of the modulated and unmodulated laser statesin the second power splitter 28. This circuit together with thephotodetector 30 effectively acts as an optical homodyne receiver wherea modulated laser signal is mixed with a CW local oscillator signal, butwithout the constraint of requiring two separate lasers.

The resulting power spectrum of the output photocurrent is composed oftwo components. The first is a direct feedthrough term associated withthe gated intensity modulation of the laser. The second term is theresult of optical mixing between the unmodulated and modulated laserstates. This second term provides for the direct observation of thefrequency excursions on the optical signal produced by the laser. Inpractice, these two terms are easily distinguished, since the intensitymodulation terms are spectrally narrow, while the terms associated withthe optical FM spectrum are broadened due to the laser linewidth.

Actual measurements on a DFB laser have been performed using the opticalspectrum analysis system 10 shown in FIG. 1. The output of a Toshiba DFBlaser (Model TOLD 300s, threshold current=14.4 mA) operating at 132 umwas fed through two isolators (not shown) before being coupled into thefirst power splitter 22. The delay difference (>3 usec) was much longerthan the coherence time of the DFB laser (linewidth >20 MHz).

The signal produced by the photodetector 30 was fed to the analyzer 32,for example, a calibrated 100 kHz to 22 GHz microwave spectrum analyzer,such as a Model HP71400A Lightwave Signal Analyzer available fromHewlett-Packard Company, Signal Analysis Division, Rohnert Park,California, comprising a high-speed InGaAs detector, microwavepreamplifier, and microwave spectrum analyzer. The bandwidth of theelectrical detection system was 22 GHz (+/-0.12 nm at 1.3 um), which iswell matched to typical frequency excursions attainable with DFB lasers.

The DFB laser was measured under both sinusoidal and PRBS modulation. Tohelp prevent thermally induced frequency chirps, the modulation source14 was AC-coupled so that the average current to the laser remainedconstant during both halves of the gate period.

FIG. 5 shows the power spectrum of the photocurrent in the photodetector30 for the cases of sinusoidal and PRBS modulation. For the sinusoidalcase (FIG. 5A), the modulation frequency was 100 MHz, the DC biascurrent was 36 mA, and the modulation index was approximately 83%. Theresulting frequency chirp of the laser was approximately +/-13 GHz,which corresponds to an FM modulation index of approximately 130. Thediscrete spectral components below 3 GHz are due to the directfeedthrough of the intensity modulation at the fundamental (100 MHz) andits various harmonics generated by non-linearities of the laser. For thePRBS case (FIG. 5B), the clock frequency was 350 MHz, the bias currentwas 50 mA, the modulation depth was about 20%, and the code sequence wasNRZ (length=2⁷ -1). The more gradual roll-off of the FM frequency chirp,compared to sinusoidal modulation, is believed to be due to the widerfrequency spectrum of the PRBS modulation. The bottom trace in FIG. 5Bis the noise floor of the analyzer 32 with the optical signal blocked.

FIG. 6 shows plots of maximum frequency chirp versus modulation index ofthe DFB laser. The modulation rates for both curves were similar (i.e.,sinusoidal=300 MHz, PRBS=365 MHz).

Accordingly, the method and apparatus in accordance with the inventioncan directly measure the homodyne power spectrum of the optical fieldfor a modulated DFB laser. Various other uses will also appear topersons skilled in the art.

The foregoing description is offered primarily for purposes ofillustration. It will be readily apparent to those skilled in the artthat numerous modifications and variations not mentioned above can stillbe made without departing from the spirit and scope of the invention asclaimed below.

What is claimed is:
 1. A method for directly measuring the optical fieldspectrum of an optical signal based on a local oscillator approach formixing down the actual optical field spectrum to baseband, comprisingthe steps of:providing an optical source; providing a modulation sourcefor selectively modulating the optical signal produced by the opticalsource so that the optical signal produced by the optical sourcealternates between (a) an unmodulated state and (b) a modulated statewhich carries the optical field spectrum of interest; routing theoptical signal produced by the optical source to an optical circuit forproviding delayed and undelayed states of the optical signal;recombining the states of the optical signal produced by the opticalsource together concurrently after being channeled through the opticalcircuit; the unmodulated state of the optical signal produced by theoptical source serving as a local oscillator signal; and mixing therecombined states of the optical signal produced by the optical source;thereby providing self-homodyne mixing in response to the unmodulatedand modulated states of the optical signal produced by the opticalsource.
 2. The method of claim 1 wherein the optical source is a laser.3. The method of claim 1 wherein the optical circuit has a differentialdelay between the delayed and undelayed states of the optical signal andthe time duration of each state equals the differential delay of theoptical circuit.
 4. The method of claim I wherein the optical circuithas a differential delay between the delayed and undelayed states of theoptical signal and the time duration of each state equals an integerfraction of the differential delay of the optical circuit.
 5. The methodof claim 1 wherein high frequency modulation is applied to the opticalsignal produced by the optical source when the modulation source isgated on by a gate function to provide gated modulation of the opticalsignal produced by the optical source.
 6. The method of claim 5 whereinthe optical circuit has a differential delay between the delayed andundelayed states of the optical signal and the modulation is at anyfrequency greater than the gate frequency and the modulation rate is nottied to the differential delay of the optical circuit.
 7. The method ofclaim 1 wherein the step of mixing the recombined states of the opticalsignal produced by the optical source comprises mixing the recombinedstates of the optical signal produced by the optical source in aphotodetector.
 8. The method of claim 7 wherein the self-homodynedmixing of the unmodulated and modulated states of the optical signalproduced by the optical source frequency-translates the optical powerspectrum to within the bandwidth of an analyzer.
 9. The method of claim8 wherein the analyzer is a microwave spectrum analyzer.
 10. An opticalspectrum analysis system, comprising:an optical source for producing anoptical signal; a modulation source connected to the optical source, themodulation source for generating a gate function employed as a triggersignal to gate a modulation signal applied to the optical signalproduced by the optical source, thereby producing a gated modulationsignal for modulating the optical signal produced by the optical sourceso that the optical signal produced by the optical source alternatesbetween (a) an unmodulated state and (b) a modulated state which carriesan optical field spectrum of interest; a first optical power splitterconnected to the optical source for splitting the alternatelyunmodulated and modulated optical signal produced by the optical source;an optical conduit having an input and an output, the input of theoptical conduit being connected to the first optical power splitter; anoptical delay line having an input and an output and a predeterminedtime delay, the input of the optical delay line also being connected tothe first optical power splitter; a second optical power splitter forrecombining the alternately unmodulated and modulated optical signalproduced by the optical source; the output of the optical conduit andthe output of the optical delay line being connected to the secondoptical power splitter; whereby the unmodulated optical signal producedby the optical source is used in lieu of a local oscillator signal. 11.The system of claim 10 wherein the optical source is a laser.
 12. Thesystem of claim 10 wherein the modulation source is a gated modulatorwhich produces bursts of modulation.
 13. The system of claim 10 whereinthe modulation signal on which the gate function is superimposed iscontinuous wave modulation.
 14. The system of claim 10 wherein theoptical circuit has a differential delay between an optical signaloutput from the optical conduit and the optical signal output from theoptical delay line and the gated modulation signal has a 50% duty cycleand a period twice the differential delay of the optical circuit. 15.The system of claim 10 wherein the relationship between the frequency ofmodulation and gate frequency of f_(m) >f_(GATE).
 16. The system ofclaim 10 wherein the optical conduit is optical fiber cable.
 17. Thesystem of claim 10 wherein the optical delay line comprises apredetermined length of optical fiber cable.
 18. The system of claim 10wherein the relationship between gate frequency and the predeterminedtime delay of the optical delay line is f_(GATE) ·T=n+1/2, where n=0, 1,2 . . . .
 19. The system of claim 10, further comprising aphotodetector, the combination of the first optical power splitter,optical conduit, optical delay line, second optical splitter, andphotodetector forming a self-homodyne receive in response to thealternately unmodulated and modulated optical signal produced by theoptical source.
 20. The system of claim 19 wherein the photodetector isa photodiode.
 21. The system of claim 19 wherein the detection bandwidthof the photodetector is wider than the AM and FM bandwidths imparted onthe optical signal.
 22. The system of claim 19 wherein, for a time T,E_(A) (t) is the unmodulated optical signal produced by the opticalsource currently serving as the local oscillator signal and E_(B) (t) isthe modulated optical signal with both AM and FM imparted on it, thesignal current I_(D) in the photodetector being proportional to E_(A)(t)² +E_(B) (t)² +2[E_(A) (t)·E_(B) (t)], the term E_(A) (t)·E_(B) (t)effectively representing the mixing between the equivalent of a localoscillator signal and the unknown signal and gives the spectralinformation of interest.
 23. The system of claim 19, further comprisingan analyzer for displaying the power spectrum of the optical signal. 24.The system of claim 23 wherein the analyzer is a microwave spectrumanalyzer.
 25. The system of claim 23 wherein the analyzer is an RFspectrum analyzer.
 26. The system of claim 10 wherein the modulationsignal on which the gate function is superimposed is pulse modulation.27. The system of claim 10 wherein the modulation signal on which thegate function is superimposed is pseudorandom bit sequence modulation.28. The system of claim 10 wherein the optical conduit is theatmosphere.